1. Field of the Invention
This invention relates to a vehicle suspension system which has a piston-cylinder device disposed between the vehicle body and each wheel and in which the suspension properties are changed by control of the hydraulic fluid fed to or discharged from the piston-cylinder device.
2. Description of the Prior Art
There has been known a so-called active suspension system which has a piston-cylinder device disposed between the vehicle body and each wheel and in which the suspension properties such as the vehicle level or hardness of the suspension are changed by control of the hydraulic fluid fed to or discharged from the piston-cylinder device. See U.S. Pat. No. 4,625,993, for instance.
In such an active suspension system, an accummulator is provided in the high pressure passage for feeding the hydraulic fluid to the piston-cylinder device from the hydraulic fluid source, and the hydraulic source normally accumulates hydraulic fluid under a predetermined high pressure in the accummulator so that the hydraulic fluid is basically fed to the piston-cylinder device from the accummulator. With this arrangement, feed of the hydraulic fluid to the piston-cylinder device can be stabilized.
However the arrangement is disadvantageous in that even after the vehicle has been stopped and the engine has been stopped, the high pressure hydraulic fluid is kept accummulated in the accummulator, which is not necessary since the suspension properties need not be controlled any more. This gives rise to the following problem.
That is, when the high pressure hydraulic fluid is kept accummulated in the accummulator even after the engine has been stopped, the accummulator and the pipings, valves and the like connected to the accummulator are kept exposed to the high pressure for a long time, which deteriorates the durability of the accummulator, the pipings, the valves and the like.
Further, when the high pressure hydraulic fluid is kept accummulated in the accummulator, the hydraulic fluid will erupt when a connection of the pipings is loosened for maintenance or the like and disturb the maintenance operation.